Various recording materials employing a coloring reaction between electron-donating dyes (leuco dyes) which have no or pale colors and color developers which are contacted with the dyes to develop colors via heat or pressure have been proposed.
Especially, thermosensitive recording material which is one group of the recording materials are disclosed in JP-B-68004160, JP-B-70014039 and JP-A-48027736. The recording process using the thermosensitive recording material is advantageous in that troublesome operations including a development and a fixation can be omitted, that it can be conducted using a relatively simple device for a short period, that a small noise is produced and that its cost is cheap. Owing to the above advantages, the thermosensitive materials have been used as the recording materials for a computer, a facsimile, a ticket vending machine, a label printer and a recorder.
In the thermosensitive material, a color former comprising a leuco dye having a lactone, lactam or spiropyran ring which is colorless or pale and a color developer comprising an organic acid or a phenolic substance are usually used. The thermosensitive material comprising the leuco dye and the color developer is preferably used because the resultant image has a high density and its background has a high whiteness.
With the increase in demands for the thermosensitive recording processes, it is strongly requested to improve storage stabilities of the recorded images to chemicals, light and heat. In this connection, the development of the thermosensitive recording materials have been strongly desired.
In order to improve the storage stability of the recorded images, various color developers have been proposed. JP-A-58082788 and JP-A-60013852 disclose phenolsulfone compounds. JP-A-61047282 and JP-A-62169681 disclose benzoic acid metal salts and substituted salicylic acid compounds, respectively. However, the thermosensitive recording material using the above compound as the color developer is not satisfactory because the recorded images have poor resistances to oils and plasticizers.
Accordingly, an aspect of this invention is to provide the thermosensitive recording material which is excellent in storage stability due to the high resistances of the recorded images to oils and plasticizers and which is excellent in matchability with a thermal head.
The provision of a new color developer suitable for the above thermosensitive recording material is other aspect of this invention.